Facilitation of noradrenergic character of sympathetic neurons by co-culturing with heart cells

Preserved left ventricular structure and function in mice with cardiac sympathetic hyperinnervation

Cardiac-specific overexpression of nerve growth factor (NGF), a neurotrophin, leads to sympathetic hyperinnervation of heart. As a consequence, adverse functional changes that occur after chronically enhanced sympathoadrenergic stimulation of heart might develop in this model. However, NGF also facilitates synaptic transmission and norepinephrine uptake, effects that would be expected to restrain such deleterious outcomes. To test this, we examined 5- to 6-mo-old transgenic (TG) mice that overexpress NGF in heart and their wild-type (WT) littermates using echocardiography, invasive catheterization, histology, and catecholamine assays. In TG mice, hypertrophy of the right ventricle was evident (+67%), but the left ventricle was only mildly affected (+17%). Left ventricular (LV) fractional shortening and fractional area change values as indicated by echocardiography were similar between the two groups. Catheterization experiments revealed that LV +/-dP/dt values were comparable between TG and WT mice and responded similarly upon isoproterenol stimulation, which indicates lack of beta-adrenergic receptor dysfunction. Although norepinephrine levels in TG LV tissue were approximately twofold those of WT tissue, TG plasma levels of the neuronal norepinephrine metabolite dihydroxyphenylglycol were fivefold those of WT plasma. A greater neuronal uptake activity was also observed in TG LV tissue. In conclusion, overexpression of NGF in heart leads to sympathetic hyperinnervation that is not associated with detrimental effects on LV performance and is likely due to concomitantly enhanced norepinephrine neuronal uptake.

Adenosine protects chick embryonic sympathetic neurons from apoptotic death by 2'-deoxyadenosine--importance of ATP in apoptosis

Our past work on nucleoside toxicity in sympathetic neurons has clearly revealed that adenosine and 2'-deoxyadenosine (dAdo) have different mechanisms of action in inducing apoptotic death. For example, adenosine is toxic to neurons only during early phase of growth whereas dAdo kills even mature neurons. In this study, we hypothesize that dAdo-induced apoptosis is initiated when ATP concentration of sympathetic neurons decreases below a critical level. To prove our hypothesis we used adenosine as a tool to replenish ATP levels of sympathetic neurons. We demonstrate that dAdo toxicity in mature sympathetic neurons was fully prevented by adenosine treatment. Furthermore, we demonstrate that depletion of ATP caused by dAdo was prevented by pretreatment with adenosine. These data suggest that intracellular accumulation of adenosine could play a neuroprotective role in preventing death associated with reduction in neuronal ATP concentration.

Adenosine induces apoptosis by inhibiting mRNA and protein synthesis in chick embryonic sympathetic neurons

Our previous work has established that adenosine is toxic to chick embryonic sympathetic neurons and kills freshly plated neurons by a process of apoptosis. Although the exact mechanism remains unknown, we found that phosphorylation of adenosine was essential to the toxicity. Using markers for RNA ([3H]uridine) and protein ([35S]methionine) synthesis we demonstrate here that in freshly plated sympathetic neurons adenosine inhibits RNA and protein synthesis by about 50%. The inhibitory effects of adenosine on RNA and protein synthesis, and increased ATP synthesis were blocked by adenosine kinase inhibitor, suggesting that phosphorylated products are responsible for inhibition of RNA and protein synthesis and cell death. Adenosine-induced inhibition of RNA and protein synthesis in neuronal cells provides a new role for adenosine in the regulation of cell function.

Role of Ca2+ in metabolic inhibition-induced norepinephrine release in rat brain synaptosomes

Ischemia and simulated ischemic conditions induce enhanced release of norepinephrine (NE) in the brain and the heart. Although studies with neuronal preparations demonstrated a rise in [Ca2+]i under energy-depleted conditions, such release of NE in the heart appears to be predominantly Ca2+ independent. Since Ca2+ overload occurs in ischemia or energy depletion and since a rise in [Ca2+]i triggers exocytosis without membrane depolarization, we tested the possibility, using brain synaptosomes, that increased NE release could be, at least in part, a consequence of raised [Ca2+]i. Brain synaptosomes were incubated with Krebs-Henseleit medium, and ischemia was mimicked by treatment with metabolic inhibitors. NE content in incubation medium (supernatant) and synaptosomes was analyzed chromatographically. Treatment with metabolic inhibitors reduced ATP content by 75% and increased [Ca2+]i by more than fourfold within minutes. Metabolic inhibition elicited NE release, which started within 10 minutes and reached a maximum after 30 minutes, with a corresponding 55% reduction in synaptosomal NE content after 40 minutes. NE release, together with a marked increase in [Ca2+]i, was also induced in energy-depleted synaptosomes by Ca2+ repletion after incubation with the Ca(2+)-free medium. Effects on NE release of various interventions to prevent Ca2+ overload were tested. Omission of Ca2+ from the incubation medium or loading synaptosomes with the Ca2+ chelator BAPTA-AM (20 and 100 mumol/L) prevented NE release, indicating a Ca(2+)-dependent mechanism. Inhibition of Ca2+ channels with omega-conotoxin, cadmium, or nifedipine had no effect on NE release during energy depletion. In contrast, nickel and 3,4-dichlorobenzamil, Na(+)-Ca2+ exchange inhibitors, dose-dependently inhibited NE release. In conclusion, this study provides evidence that under energy-depleted conditions, Ca2+ overload in synaptosomes of noradrenergic neurons from the brain is an important mechanism for the enhanced release of NE and that a reversal of Na(+)-Ca2+ exchange may be the key pathway leading to intraneuronal Ca2+ overload.

Massive exocytosis triggered by sodium-calcium exchange in sympathetic neurons is attenuated by co-culture with cardiac cells

Entry of Ca2+ through voltage-dependent Ca2+ channels is known to be linked to the exocytotic release of transmitter from sympathetic neurons. In this paper we provide evidence that transmitter release can also be stimulated by Ca2+ influx via the Na-Ca exchanger. Furthermore, the release linked to Na-Ca exchange is regulated by cardiac target cells. Cultured sympathetic neurons of the chick embryo incubated in Ca2(+)-Mg(2+)-free Krebs solution for 20 min and then switched to Ca(2+)-containing solution exhibited 15-20-fold increases in [3H]noradrenaline release over the spontaneous release. Electrophysiologic studies showed that neurons were completely depolarized in Ca(2+)-Mg(2+)-free medium. Indo-1 fluorescence revealed a large and sustained increase in intracellular free Ca2+ concentration ([Ca2+]i) after addition of Ca2+ to Ca(2+)-Mg(2+)-free medium. The increased [3H]noradrenaline release and [Ca2+]i were dependent on external Na+ and Ca2+, but were not affected by the Ca2+ channel blockers lanthanum, cadmium, verapamil or omega-conotoxin. A conventional depolarizing stimulus (125 mM K+) produced a 13-fold increase in [3H]noradrenaline release over spontaneous release. However, K(+)-induced release and rise in [Ca2+]i declined rapidly and were sensitive to the Ca2+ channel blockers. When sympathetic neurons were co-cultured with embryonic cardiac cells the release induced by change from Ca(2+)-Mg(2+)-free to Ca(2+)-Krebs solution was dramatically reduced. The change from Ca(2+)-Mg(2+)-free to Ca(2+)-Krebs solution was ineffective in evoking [3H]noradrenaline release from sympathetic neurons in situ using perfused hearts of 15-day-old chick embryos.(ABSTRACT TRUNCATED AT 250 WORDS)

A monoclonal anti-glycoconjugate antibody defines a stage and position-dependent gradient in the developing sympathoadrenal system

The expression of complex carbohydrate antigens was analysed in developing sympathoadrenal cells of the rat using monoclonal antibodies that react with unique carbohydrate structures. CC1 and CC4 are monoclonal antibodies that react specifically with beta-N-acetylgalactosamine and alpha-galactose/alpha-fucose moieties, respectively. CC1-reactive glycoconjugates are expressed in embryonic superior cervical ganglion (SCG) cells as early as embryonic day 15 (E15). CC4 is expressed in the SCG only for a brief period starting at E18 and then disappearing at P5. During their transient period of expression, CC1 antigens are expressed uniformly throughout the SCG at E15-17, but are then restricted to the rostral portion of the SCG from E18 to P4. CC4 is also concentrated in the rostral portion of the SCG between E21 and P4. In the adrenal medulla, CC1 and CC4 antigens display a post-natal onset of expression commencing approximately at P14 and continue to be expressed on a subset of cells which contain tyrosine hydroxylase (TH). The expression of CC1, however, is restricted to phenylethanolamine-N-methyltransferase-(PNMT)-negative chromaffin cells, whereas CC4 is not. CC1 and CC4-expressing cells appear to be scattered throughout the adrenal medulla without any particular topographic orientation. These findings suggest that the CC1 monoclonal antibody defines a stage-specific differentiation antigen in the sympathoadrenal lineage. Additionally, the CC1 antigen may confer important positional information in the embryonic SCG by distinguishing rostral from caudal neuronal cell bodies.

Neurotransmitter plasticity in the sympathetic nervous system: influence of external factors and possible physiological implications

Neuronal function can be modulated by a variety of neuronal, environmental and hormonal stimuli. One form of neuronal modulation is the change in the biosynthesis of specific neurotransmitters. This is of particular interest since neurotransmitters are the agents responsible for neuronal communication. The analysis of the long-term modulation of neurotransmitter expression in response to external factors could be a suitable model to study the possible biochemical mechanisms involved in learning and memory. Furthermore, understanding the molecular mechanisms involved in the regulation of norepinephrine synthesis in the sympathetic nervous system may be relevant for understanding stress and diseases of the cardiovascular system.

Forskolin mediates the survival of nerve growth factor-dependent sympathetic neurons of chick embryo by a cyclic AMP-independent mechanism

Forskolin has become an invaluable tool for exploring the involvement of cyclic AMP in a variety of cellular functions. The diterpine directly activates the catalytic subunit of adenylate cyclase, causing a marked increase in cyclic AMP content. Because of this well-characterized action, practically all the observed effects of forskolin are commonly attributed to cyclic AMP-dependent processes. We show here that forskolin exerts a neurotrophic action that is almost identical to that of nerve growth factor (NGF) and phorbol 12,13-dibutyrate (PDB) but independent of cyclic AMP. Sympathetic neurons of the chick embryo supported in culture for 2 days by NGF, forskolin plus 3-isobutyl-1-methylxanthine (IBMX), or PDB had almost identical levels of cyclic AMP (between 9 and 12 pmol/mg protein). Neurons supported in culture for 2 days by NGF or PDB when challenged with forskolin plus IBMX showed almost a 15-fold increase in cyclic AMP, but those supported by forskolin plus IBMX and then exposed to the same combination of drugs did not show an increase in cyclic AMP, exhibiting a marked down-regulation. Exposure of neurons to forskolin for 2 h was ineffective in supporting long-term survival, suggesting that an initial increase in cyclic AMP formation is not sufficient but the continued presence of the drug is essential for survival. Effects of forskolin on the survival of these neurons could be observed even in the presence of dideoxyadenosine, and inhibitor of adenylate cyclase. Neurons supported by PDB for 2 days in culture when exposed to NGF for the first time did not show any increase in cyclic AMP, providing clear evidence that NGF does not affect this second messenger in its target cells. Similarly, neurons supported by NGF for 2 days when exposed to PDB did not show an increase in cyclic AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Veratrine supports the in vitro survival of embryonic chick sympathetic neurons

Veratrine (VT), an alkaloid known to act on the sodium channels and cause depolarization of a cell membrane, was found to support the survival of cultured sympathetic neurons. At 30 microM it was as effective as nerve growth factor (NGF), as determined by the cell counts and [3H]norepinephrine ([3H]NE) uptake. Protein kinase C (PKC) activity of the surviving neurons was measured because of our previous finding that depolarizing concentrations of K+ support the survival and cause several fold increase in the enzyme activity. An acute treatment of NGF-supported sympathetic neurons by VT did not alter PKC activity.

Co-localization and plasticity of transmitters in peripheral autonomic and sensory neurons

Immunohistochemical studies have shown that most peripheral autonomic and sensory ganglia are heterogeneous, consisting of several populations of neurons which can be distinguished by their content of peptide and non-peptide transmitters, and transmitter-associated enzymes. Many neurons contain several different potential transmitters, especially neuropeptides. Some neuropeptides have been localized in more than one population of autonomic and sensory neurons. However, the peptide often occurs together with a distinctive combination of additional transmitters in each neuronal class. The precise combination of transmitters found in any individual neuron is highly correlated with the peripheral target of the neuron. This indicates that immunohistochemically defined neuronal populations represent distinct functional classes of neurons. In an increasing number of cases, many of the potential transmitters contained in a particular neuron have been shown to be released from the nerve terminals, and to contribute to presynaptic or postsynaptic effects of nerve activation. Despite this association between the combination of potential transmitters contained in a neuron, and the function of the neuron, not all transmitters or transmitter-associated enzymes are expressed equally at all times in the life of a neuron: the levels of some substances change dramatically during development; some are detected only after experimental alteration of the environment of the developing or mature neurons. Taken together, these results indicate that, during development, pathway-specific information influences the differentiation of peripheral autonomic and sensory neurons. Furthermore, the expression of neuropeptides and transmitter-associated enzymes in a particular neuron appears to be under continuous regulation. These phenomena demonstrate the complexity and precision involved in development and maintenance of the peripheral autonomic and sensory nervous systems.

Demonstration of adrenergic and dopaminergic receptors in cultured sympathetic neurons--their coupling to cAMP but not to the transmitter release process

Experiments were carried out on cultured sympathetic neurons of the chick embryo; first, to demonstrate the presence of adrenergic and dopaminergic receptors, and then to see if these receptors are involved in regulation of transmitter release. We show that alpha 2-agonists, norepinephrine, epinephrine and clonidine, had no effect on neuronal cyclic 3',5'-adenosine monophosphate content. Forskolin enhanced neuronal cyclic 3',5'-adenosine monophosphate from a control value of about 20 pmoles/mg protein to 150 pmoles/mg protein. In the presence of alpha 2-agonists and forskolin the cyclic 3,5'-adenosine monophosphate content increased between 340 and 430 pmoles/mg protein. The alpha 1-agonist, phenylephrine, had no such effect. The facilitatory effect of alpha 2-agonist on forskolin-stimulated cyclic 3',5'-adenosine monophosphate production was blocked by the alpha 2-antagonist, yohimbine, but not the alpha 1-agonist, prazosin. Dopamine did not affect neuronal cyclic 3',5'-adenosine monophosphate content, but forskolin-stimulated increase in cyclic 3',5'-adenosine monophosphate was further facilitated by dopamine, and this effect was blocked by haloperidol. Activation of neuronal alpha 2-receptors by norepinephrine, epinephrine and clonidine did not interfere with electrically induced release of tritium from [3H]-norepinephrine-loaded sympathetic neurons. However, if sympathetic neurons were co-cultured with heart cells, clonidine, norepinephrine and epinephrine markedly inhibited the stimulation-induced release. Yohimbine or phentolamine partially reversed the inhibitory effects of alpha 2-agonists. alpha 2-Agonists and -antagonists also modified stimulation-induced release of tritium from [3H]norepinephrine-loaded hearts of the chick embryo.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of transmitter release properties of embryonic sympathetic neurons growing in vivo and in vitro

The functional behavior of embryonic chick sympathetic neurons was determined by inducing release of [3H]norepinephrine by electrical stimulation of sympathetic neurons growing in the chick heart and in culture, with and without heart cells. A very close correspondence between the functional behavior of neurons developing with the heart cells, either in vivo or in vitro, was demonstrated. For example, the outflow of tritium from [3H]norepinephrine loaded sympathetic neurons of 15-day-old chick heart was about three times more at 10 Hz than at 1 Hz. In contrast, the outflow of tritium from 12-day-old [3H]norepinephrine loaded cultured sympathetic neurons was inversely related to the frequency of stimulation (outflow at 1 Hz was about three time more than at 10 Hz). When neurons were co-cultured with the heart cells, the frequency-outflow relationship reverted to that seen in the intact heart. Electrically-evoked outflow of tritium from the heart was reduced in a concentration-dependent manner by 3-30 nM tetrodotoxin, abolished in 0.25 mM Ca medium, and potentiated by 3 mM tetraethylammonium. In sharp contrast, the outflow evoked by stimulation of cultured neurons was neither blocked by 30-300 nM tetrodotoxin, low Ca, nor potentiated by tetraethylammonium. However, when neurons were co-cultured with heart cells, the evoked outflow was blocked by 30 nM tetrodotoxin and low Ca, and potentiated by tetraethylammonium. Veratrine (10 microM) had very little effect on the outflow from cultured neurons but induced a massive outflow from co-cultures as well as hearts. Neurons grown in a medium conditioned by the heart cells were not sensitive to tetrodotoxin and veratrine. It is implied that cultured sympathetic neurons are endowed mostly with Ca channels, and that the Na channels become functional only when neurons are grown with the target cells. This dramatic alteration in the functional behavior of neurons co-cultured with heart cells indicates that the effector organ has an important role in the development of ionic conductances of sympathetic neurons growing in the body and in culture.